The recent trend for portable apparatus has increased the requirement for high energy density rechargeable batteries. High energy density is also important for batteries used for electric vehicles.
Nickel hydroxide has been used for years as an active material for the positive electrode of alkaline electrochemical cells. Examples of such nickel-based alkaline cells include nickel cadmium (Ni--Cd) cells and nickel-metal hydride (Ni-MH) cells. The energy density of such nickel-based electrochemical cells may be increased by closely packing the nickel hydroxide active material into an electrically conductive substrate such as a porous foam. However, there are limitations on the amount of pressure used to increase packing density. Application of too much pressure causes expansion of electrode plates and compresses the separators placed between the positive and negative electrodes. The compression of the separators presses out the electrolyte solution and deteriorates the discharge characteristics.
In a nickel cadmium cell, cadmium metal is the active material in the negative electrode. Ni--Cd cells use a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in the alkaline electrolyte. The charge/discharge reactions at the negative electrode are controlled by the following reaction: ##EQU1##
During charge, electrons are supplied to the negative electrode, whereby Cd(OH).sub.2 is reduced to Cd. During discharge, Cd is oxidized to Cd(OH).sub.2 and electrons are released.
The reactions that take place at the positive electrode of a Ni--Cd cell are also reversible. For example, the reactions at a nickel hydroxide positive electrode in a nickel cadmium cell are: ##EQU2## At the positive electrode, Ni(OH).sub.2 is oxidized to NiOOH during the charge operation. During discharge, the NiOOH is reduced to Ni(OH).sub.2.
In general, nickel-metal hydride (Ni-MH) cells utilize a negative electrode comprising a metal nydride active material that is capable of the reversible electrochemical storage of hydrogen. Examples of metal hydride materials are provided in U.S. Pat. Nos. 4,551,400, 4,728,586, and 5,536,591 the disclosures of which are incorporated by reference herein. The positive electrode of the nickel-metal hydride cell comprise a nickel hydroxide active material. The negative and positive electrodes are spaced apart in the alkaline electrolyte.
Upon application of an electrical potential across a Ni-MH cell, the Ni-MH material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical generation of hydroxyl ions: ##EQU3## The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron.
The reactions that take place at the nickel hydroxide positive electrode of a Ni-MH cell are the same as for a Ni--Cd cell and are provided by reaction (2).
Hence, the charging process for a nickel hydroxide positive electrode in an alkaline storage battery is governed by the following reaction: ##EQU4##
The charging efficiency of the positive electrode and the utilization of the positive electrode material is affected by the oxygen evolution process which is controlled by the reaction: EQU 2OH--.fwdarw.H.sub.2 O+1/2O.sub.2 +2e- (5)
During the charging process, a portion of the current applied to the battery for the purpose of charging, is instead consumed by the oxygen evolution reaction (5). The oxygen evolution of reaction (5) is not desirable and contributes to lower utilization rates upon charging. One reason both occur is that their electrochemical reaction potential values are very close. Anything that can be done to widen the gap between them (i.e., lowering the nickel reaction potential is reaction (4) or raising the reaction potential of the oxygen evolution reaction (5)) will contribute to higher utilization rates. It is noted that the reaction potential of the oxygen evolution reaction (5) is also referred to as the oxygen evolution potential.
Furthermore, the electrochemical reaction potential of reaction (5) is temperature dependent. At lower temperatures, oxygen evolution is low and the charging efficiency is high. However, at higher temperatures, the electrochemical reaction potential of reaction (5) decreases and the rate of the oxygen evolution reaction (5) increases so that the charging efficiency of the nickel hydroxide positive electrode drops. High temperatures at the positive electrodes may be due to the external environment at which the battery is operated. They may also be due to the heat generated within the battery by oxygen gas recombination at the negative electrodes.
One way to increase the reaction potential of equation (5) is by mixing certain additives with the nickel hydroxide active material when forming the positive electrode paste. U.S. Pat. Nos. 5,466,543, 5,451,475 and 5,571,636 disclose certain additives which improve the rate of utilization of the nickel hydroxide in a wide temperature range. The present invention discloses new additives which improve the high temperature utilization of nickel-based positive electrode materials.